Heretofore, metallic materials and processed products thereof which are chemically active and, therefore, are easily oxidizable in the air have been posing various problems due to the occurrence of corrosion or rust. Great technical efforts have been being made for the solution of these problems
In the case of a permanent magnet made of an intermetallic compound using rare earth-iron or rare earth-cobalt as a main component thereof, for example, the surface of the magnet rusts because of the compound's easy oxidizability and the formation of the oxide on the magnet's surface degrades the output of the magnetic circuit and the oxide shed from the surface smears devices disposed nearby. Thus, the rust entails various problems.
Techniques have been heretofore proposed for forming on the surfaces of magnets and other similar metallic articles varying corrosionproof and rustproof coatings. To be specific a method which forms an antioxidant forming coating by the use of a phosphate, a chromate etc. [Japanese Patent Laid-Open SHO 60(1985)-240,105], methods which form a coating of such metal as Zn by vacuum evaporation, ion plating, etc. [Japanese Patent Laid-Open SHO 61(1986)-185,910 and Japanese Patent Publication SHO 55(1980)-34,570], and methods which form a resinous coating by the use of resinous materials such as acryl resin and epoxy resin [Japanese Patent Laid-Open SHO 59(1984)-162,239 and Japanese Patent Laid-Open SHO 59(1984)-163,806] have been known in the art.
The inventor's test on permanent magnets provided with the aforementioned corrosionproof coatings has revealed that though these coatings are invariably effective in rustproofing the magnets to a certain extent the degrees of effect are not necessarily sufficient and the coatings are incapable of retaining a rustproofing effect long under the impact of aging.
Further, the forming coatings produced by the use of a phosphate, a chromate etc. have the possibility of deteriorating the physical properties of the coated metals and degrading the performances of the metals. In the case of the magnet mentioned above for example the passivation of the magnet's surface gives rise to a nonmagnetic altered layer, which goes to deteriorate the magnet's properties, etch the components of the magnetic powder alloy, and weaken the magnet's strength.
The method which consists in forming a coating of such metal as Zn has the problem that the device used for the formation of this coating is complicated and bulky and the method itself is deficient in operational efficiency, incapable of mass-production, and apt to prove expensive. The method which consists in forming a resinous coating requires the coating to be formed in a large thickness in due consideration of the possible occurrence of a porous texture in the applied layer of resin and the thick coating of resin consequently formed has the possibility of inconveniencing the application for which the coated article is intended. In the case of a permanent magnet of the kind described above, for example, when the magnet is used as a rotor magnet for a motor, the motor is inevitably required to interpose a large air gap between the rotor and the stator at an inevitable sacrifice of motor torque.
An object of this invention is to provide a novel corrosionproof coating material and a method for the formation of a coating therewith. Another object of this invention is to provide a corrosionproof coating material possessing a highly prominent rustproofing effect and a method for the formation of a coating therewith. Still another object of this invention is to provide a corrosionproof coating material capable of manifesting a rustproofing effect without impairing the characteristics of a metal being coated and a method for the formation of a coating therewith. Yet another object of this invention is to provide a corrosionproof coating material fit for mass production at a low cost and for formation of a thin film and a method for the formation of a coating therewith. A further object of this invention is to provide a corrosionproof coating material usable advantageously for coating the surface of a rare earth-iron-boron type permanent magnet and a method for the formation of a coating therewith.